JPS641559B2 - - Google Patents

Description

[Detailed description of the invention]

(Field of Industrial Application) The present invention relates to a method of electrodeposition coating using a cationic electrodeposition paint, and is particularly useful for suppressing the occurrence of convex coating films and traces of contamination, and preventing coating defects. This invention relates to a cationic electrodeposition coating method. (Prior art) The electrodeposition coating method is a method in which a coating film is deposited on the surface of the object to be coated through a process such as electrophoresis by applying a voltage, but it depends on the voltage application method and the type of paint used. Generally, they are divided into cationic electrodeposition coating methods and anionic electrodeposition coating methods. Cationic electrodeposition coating methods are mainly used for painting automobile bodies. Further, an electro-deposition coating line for car bodies usually consists of various steps in the order of degreasing, washing with water, surface conditioning, chemical conversion treatment, washing with water, drying, electro-deposition painting, washing with water, and baking. (Problems to be Solved by the Invention) However, in the conventional cationic electrodeposition coating method, coating film defects such as convex coating films, so-called bumps, and traces of contamination may occur. Painting of automobile bodies is generally completed by applying an undercoat using electrocoating, followed by an intermediate coat and a topcoat. However, if defects such as spots occur during the electrocoating process, these defects may be left as is and the entire painting process is completed. When the process is completed, the paint film defects remain, resulting in a completed automobile body with defective paint. Therefore, conventionally, when a coating film defect occurs, it has been necessary to smooth the convex coating film by dry sanding after electrodeposition coating. Therefore, there has been a strong demand for an electrodeposition coating method that does not cause any coating defects. Various proposals have been made to prevent the above-mentioned coating defects. For example, as a recent proposal, as described in Japanese Patent Application Laid-Open No. 56-47595,
Examples include a method in which the object to be coated after chemical conversion treatment is treated with an aqueous acid solution such as formic acid having a pH of 2 to 7, then washed with deionized water, and then dried. However, although this method is effective in removing traces of contamination from chemical conversion treatment liquids, etc., acid components are not completely removed from the surface of the object to be coated by washing with deionized water, and the effects of this residual acid Therefore, there has been a problem in that rust or corrosion of the object to be coated may occur during the subsequent electrodeposition coating process. In addition, in Japanese Patent Publication No. 47-51094, the coating layer treated with phosphoric acid coating becomes porous, and the processing solution from phosphoric acid coating processing remains inside, and the processing solution can be removed by ordinary spraying or showering. It cannot be removed by cleaning treatments such as phosphoric acid coatings, and it remains inside the coating layer and causes paint deterioration. However, water or paint solution must be sprayed on the surface immediately before sending it to the coating tank, and even then, water or paint solution must be sprayed immediately before sending the paint to the paint tank, and even then the wettability and conductivity of the surface to be painted will be affected, resulting in poor paint finish. Disclosed that it is not effective because it causes pinholes and other undesirable results due to insufficient phosphoric acid coating, and as a means of removing the treatment solution remaining in the phosphoric acid coating layer, the phosphoric acid coated body is immersed in a water tank. It is proposed that ultrasonic irradiation be performed. In addition, the main cause of film defects in electrodeposition coating is that when a voltage is applied in the paint while a foreign substance with a resistance different from that of the chemical conversion coating is attached to the surface of the object to be coated, the conductivity of the part where the foreign substance is attached becomes different. It is thought that this is due to the fact that excessive electrolysis progresses at the adhesion point, causing a large amount of paint to deposit, forming a convex coating film. Apart from the above-mentioned problem of the treatment liquid remaining inside the phosphoric acid coating layer, the present inventors have discovered that many of the foreign substances that adhere to the car body in the electrodeposition coating line for automobile bodies are due to zinc phosphate used in chemical conversion treatment. It was confirmed that it was a crystalline powder by analyzing the surface of the car body in the middle of electrodeposition painting. And, this adhesion of zinc phosphate is, for example,
During the water washing and drying process after chemical conversion treatment, the chemical conversion treatment solution (containing zinc phosphate) falls downward, and as a result of subsequent drying, it becomes crystalline powder of zinc phosphate, etc., and is deposited on the floor of the drying oven. It is thought that the powder accumulates, and then this powder is blown up by the hot air in the drying oven during transportation of the car body and adheres to the surface of the car body just before the electrodeposition coating. Therefore, it is considered effective to perform a suitable treatment immediately before electrocoating when painting an automobile body in order to remove the attached foreign matter, that is, to prevent defects in the paint film. An object of the present invention is to provide a cationic electrodeposition coating method that can suppress the occurrence of coating film defects such as spots and traces of contamination and can obtain a good coating appearance. (Means for Solving the Problems) As a result of intensive research to suppress the occurrence of coating film defects in the above-mentioned cationic electrodeposition coating, the present inventors found that after undergoing water washing and drying treatment after chemical conversion treatment, When deionized water with a pH of 5.8 to 8.2 and a specific conductivity of 10μ/cm or less is sprayed onto the object using a spray gun, etc., and then subjected to cationic electrodeposition, there are no defects such as convex coatings. discovered that an excellent painted appearance could be obtained, and completed the present invention. That is, the cationic electrodeposition coating method of the present invention is as follows:
After the pre-painting process of degreasing, chemical conversion, and subsequent washing and drying, deionized water with a pH of 5.8 to 8.2 and a specific conductivity of 10μ/cm or less is sprayed onto the object, followed by cationic electrolysis. It is characterized by the fact that it is coated with paint. When deionized water with a pH of 5.8 to 8.2 and a specific conductivity of 10μ/cm or less is sprayed onto a workpiece that has undergone the prescribed pre-painting process, and then cationic electrodeposition is applied, a convex coating film (butts) is formed. ) or any trace of contamination can be obtained, and the occurrence of coating defects can be effectively suppressed. On the other hand, if the pH is less than 5.8 or
Deionized water greater than 8.2 or specific conductivity 10μ/cm
When the object to be coated is sprayed with deionized water exceeding 100 ml of deionized water after the previous process is completed in the same manner as above, and then subjected to cationic electrodeposition coating, a convex coating film, etc. will sometimes appear on the surface of the object, albeit slightly. Prevention of coating film defects becomes incomplete. Therefore, the electrocoating line for automobile bodies to which the method of the present invention is applied has a structure in which the car body is washed with deionized water after going through the usual chemical conversion treatment process, and then cationic electrocoating is performed. Become something. This consists of steps in the following order, for example. Hot water washing treatment → spray degreasing treatment → immersion degreasing treatment → first water washing treatment → second water washing treatment → surface conditioning treatment → immersion chemical conversion treatment → third water washing treatment → immersion water washing (fourth washing) treatment → fifth washing treatment → draining Drying treatment → deionized water spraying treatment → cationic electrodeposition coating → water washing treatment → baking treatment. In addition, in the present invention, it is possible to completely prevent the occurrence of convex coating films, etc., by spraying the object to be painted with the specified deionized water, which removes foreign substances, especially phosphorus, remaining on the surface of the object to be painted. This is thought to be due to the fact that the acid zinc powder is dissolved or washed away in deionized water, and the object to be coated from which all foreign matter has been removed can be subjected to cationic electrodeposition coating. (Example) Examples of the present invention will be described below, but the coating method of the present invention is not limited to these examples, and various changes and modifications may be made without departing from the gist of the present invention. Of course you can get it. First, test pieces used in Examples and Comparative Examples were manufactured. A zinc phosphate-based chemical conversion coating was formed on the surface of a 150 mm x 80 mm x 0.8 mm mild steel plate according to the following process order, and a test piece was manufactured. (1) Degreasing → (2) Washing with water → (3) Chemical conversion treatment → (4) Washing with water → (5) Drying As the chemical conversion treatment agent, commercially available Granodin SD-2000 manufactured by Nippon Paint Co., Ltd. was used. Example 1 Deionized water (PH5.8-8.2, specific conductivity 10μ/cm)
was sprayed onto the above test piece using a commercially available spray gun (Waida 70 model: manufactured by Iwata Yakinki Kogyo Co., Ltd.). The spray amount was 100ml/ ^m2 . Thereafter, an electrodeposition coating film is formed on the surface of the test piece according to the following conditions for cationic electrodeposition coating. (1) Conditions for cationic electrodeposition coating (a) Electrodeposition coating: Cationic electrodeposition paint (Svia
CED#50M Gray Made by Shinto Paint Co., Ltd.
Other Nippon Paint Co., Ltd. products or Kansai Paint Co., Ltd. products may also be used. ). (b) Painting conditions Paint temperature: 29℃, anode: stainless steel plate, cathode: mild steel plate Distance between electrodes: 250mm, voltage between electrodes;
200V bath entry speed; 3cm/sec, total immersion time;
3 minutes (c) Deposited film thickness: 20 to 25μ (d) Other conditions follow conventional methods. Thereafter, the coated test piece is washed with deionized water, and then baked for 20 minutes in a conventional hot air drying oven (temperature inside the oven: 170°C) to complete the coated article. Comparative Example 1 The above test piece produced in the preliminary process was subjected to cationic electrodeposition coating without being sprayed with deionized water to form an electrodeposited film on the surface of the test piece, and then the test piece was washed with water and subjected to baking treatment. . The conditions for electrodeposition coating, washing with water, and baking treatment were kept the same as in Example 1. Comparative Examples 2 to 4 Acidic water with a pH of less than 5.8, alkaline water with a pH of more than 8.2, and deionized water with a specific conductivity of more than 10 μ/cm were used to spray the test piece prepared in the preliminary process, Subsequently, each test piece was subjected to cationic electrodeposition coating, water washing, and baking treatment in the same manner as in Example 1. The painted appearance of the painted test pieces obtained in Example 1 and Comparative Examples 1 to 4 above was visually inspected under a white light bulb. The results are shown in Table 1.

[Table] Example 2 The method of the present invention was applied to an actual electrodeposition coating line for automobile bodies. Normal pre-painting process, namely hot water washing → spray degreasing → immersion degreasing → first water washing → second water washing → surface conditioning treatment → immersion chemical conversion treatment →
After going through the steps of 3rd water washing → immersion washing → 5th water washing → draining and drying, as shown in Figure 1, the pH is 5.8 to 8.2 and the specific conductivity is 10μ/cm.
The following deionized water 1... is sprayed onto the surface of the car body 2 to be painted using spray guns 3a to 3c. The spray guns 3a to 3c (manufactured by Wider Spray Gun Iwata Painting Equipment Co., Ltd.) are each equipped with a dry auger nozzle 4 (manufactured by Ikeuchi Co., Ltd.) at their tips, and the spray gun 3a is located at the center of the top surface of the vehicle body 2. spray gun 3b is for car body 2
The upper end portion and the spray gun 3c are for the side surface of the vehicle body 2. The amount of deionized water sprayed is 100ml/
^m2 . Thereafter, the car body 2 is coated with cationic electrodeposition using an electrodeposition paint (Svia CED#50M Gray Shinto Paint Co., Ltd.), and then the car body 2 is washed with water and subjected to a baking treatment. The conditions for electrodeposition coating, water washing, and baking treatment were the same as in Example 1. Comparative Example 5 In an actual car body painting line, the conditions for other treatments such as degreasing, chemical conversion treatment, and cationic electrodeposition coating were the same as in Example 2, without performing deionized water spray treatment immediately before painting. An electrodeposition coating is formed on the surface of an automobile body. The painted appearance of the painted vehicle bodies obtained in Example 2 and Comparative Example 5 was then examined in the same manner as above. The results are shown in Table 2.

[Table] From Table 1 and Table 2, Example 1 and Example 2
It can be seen that the method described above suppresses the occurrence of convex coating films, etc. (Effects of the Invention) As explained above, the cationic electrodeposition coating method of the present invention uses specific deionized water to perform a spray treatment immediately before the electrodeposition coating after a predetermined pre-process. This method is completely free of defects such as convex coatings (bumps), bulges in the coating, and traces of contamination, and has an extremely excellent painted appearance. Moreover, this method does not involve the use of acid solutions to remove foreign substances. , rust and corrosion of painted objects caused by insufficient acid removal do not occur at all.

[Brief explanation of drawings]

FIG. 1 is a diagram showing a deionized water spraying process according to the method of Example 2 of the present invention. In the figure, 1...deionized water.

Claims (1)

[Claims] 1 Degreasing treatment, chemical conversion treatment, and subsequent washing with water,
After the pre-painting process of drying treatment, deionized water with a pH of 5.8 to 8.2 and a specific conductivity of 10μ/cm or less is sprayed onto the object to be coated, followed by cationic electrodeposition. How to apply the paint.